Semiconductor laser device

ABSTRACT

A semiconductor laser according to the present invention comprises a λ/2 dielectric film (λ: in-medium wavelength of a dielectric film, for example, SiO 2 , Si 3 N 4 , Al 2 O 3 , and AlN) in contact with a facet of a resonator; and a first dielectric double layered film disposed on the dielectric film, which includes a first layer of a-Si and a second layer of a material having a refractive index lower than that of a-Si. The first layer has a thickness ¼ of an in-medium wavelength of a-Si, and the second layer has a thickness ¼ of a in-medium wavelength of the second layer. Therefore, it is possible to firmly stack the first dielectric double layered film and form a high reflectance film with high yield.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor laser device, and moreparticularly, to a semiconductor laser device used for opticalcommunication, having a wavelength of 1250 nm or more and provided witha dielectric film on a resonator facet of a semiconductor laser.

2. Description of the Related Art

As the amount of communication demand drastically increases, efforts arebeing made to realize large capacity communication systems. Transmissionsystems in optical communication mainly use 1.3 μm band signal light and1.55 μm band signal light.

The 1.55 μm band signal light involves only small optical fiber loss andis used as signal light in a long-distance communication system. This iscalled “interurban communication (trunk system)” and used forcommunication between megalopolises, for example, between Tokyo andOsaka.

On the other hand, the 1.3 μm band signal light involves greater opticalfiber loss but has less wavelength dispersion and is used as signallight in a short-distance communication system. This is called, forexample, “intra-urban communication” and used for communication within amegalopolis. Furthermore, the 1.3 μm band signal light is also used forcommunication between a base station and individual households called an“access system.”

Long-wavelength semiconductor lasers that generate signal light having awavelength of 1.25 μm or more including such signal light are alsorequired to perform high-speed response at a low operating current.

Facet emitting semiconductor lasers are normally used as long-wavelengthsemiconductor lasers for optical communication. An facet emittingsemiconductor laser generally has a pair of mutually opposed facetsformed in a crystal by cleaving or etching, allows light to travelbetween the facets through reflection and thereby obtains light feedbacknecessary for laser oscillation. Such a semiconductor laser is known asa Fabry-Perot semiconductor laser.

In addition, lasers such as a distributed feedback semiconductor laserusing a diffraction grating and distributed reflector semiconductorlaser are known, and many such lasers have a structure using reflectionon an facet in addition to light feedback using a diffraction grating.

For example, the semiconductor laser body of a Fabry-Perot semiconductorlaser has a resonator having a multilayered structure of an n-type cladlayer, active layer and p-type clad layer. A single-layer film made ofAl₂O₃ and having a thickness of λ/2 is formed on the front facet of thisresonator.

Here, λ is an in-medium wavelength and defined as λ=(wavelength invacuum of light that emits from the semiconductor laserbody)÷(refractive index of medium through which laser light propagates).

When a single-layer film made of Al₂O₃ and having a thickness of λ/2 isformed on the facet of the resonator, the reflectance of light on thefacet is determined by a refractive index of the semiconductor making upthe resonator with respect to air. For example, when the refractiveindex of the semiconductor is 3.2, the reflectance is on the order of30%.

Furthermore, a multilayer film made up of, for example, an SiO₂ film andSi film is formed on a rear facet of the resonator. In the case of amultilayer film composed of three layers of an SiO₂ film, Si film andSiO₂ film, each layer having a thickness of λ/4, the reflectance becomesapproximately 60%. When two layers of a Si film and SiO₂ film areadditionally stacked outside the SiO₂ film, the reflectance becomesapproximately 90%.

Furthermore, in the case of a distributed feedback semiconductor laser,for example, a diffraction grating is provided along an active layer ofthe resonator and an antireflection film is formed on the front facetand a high reflection film similar to a Fabry-Perot semiconductor laseris formed on the rear facet.

Laser oscillation is generally started when a current equal to or higherthan a predetermined value is passed through a semiconductor laser. Thecurrent value in this case is called a “threshold current.” Thethreshold current is a current that does not contribute to lightemission of the laser and is generally preferred to be low. Thethreshold current corresponds to a current value at which a gainproduced by current injection is balanced with resonator loss. Here, theresonator loss is the sum of internal loss (absorption loss or the like)and mirror loss.

The mirror loss is greater as the reflectance of the facet is lower, andtherefore increasing the reflectance of the facet causes the mirror lossto decrease and can reduce the threshold current.

Furthermore, in the case of a distributed feedback semiconductor laseror distributed reflector semiconductor laser, the degree of influence ofa diffraction grating on light (normalized coupling coefficient) alsohas a large influence on mirror loss and the facet reflectance also hasa large influence. However, as for the normalized coupling coefficient,increasing the normalized coupling coefficient allows mirror loss to bereduced.

However, increasing the reflectance of the facet of any semiconductorlaser causes light density on the facet to increase.

For example, with regard to a Fabry-Perot semiconductor laser, the lightdensity on the front facet of a Fabry-Perot semiconductor laser having afront facet reflectance of 60% and rear facet reflectance of 90% isapproximately double that of a Fabry-Perot semiconductor laser having afront facet reflectance of 30% and rear facet reflectance of 60% whenlight outputs are the same.

Furthermore, in a distributed feedback semiconductor laser using anantireflection film for the front facet and a high reflection film forthe rear facet, though the light density also depends on the phase ofthe diffraction grating on the rear facet, there are many device whoselight density on the rear facet increases. This tendency becomesnoticeable especially when the normalized coupling coefficient isincreased in order to reduce a threshold current or when the reflectanceof the rear facet is increased, and when assuming that the normalizedcoupling coefficient is 1.4, the front facet reflectance is 0% and therear facet reflectance is 90%, the light density on the rear facet isapproximately seven times that of the front facet at maximum.

On the other hand, the interface between the semiconductor and the facetcoating film is generally a location where there are many interfacestates and laser deterioration is most likely to occur, and if the facetcoating film is designed so that the electric field strength of light atthis location becomes a maximum, such an interface is likely to causedeterioration of the laser.

For example, in the case of a resonator of a Fabry-Perot semiconductorlaser, a high reflection film is disposed on the rear facet of theresonator as follows. That is, this is a high reflection film in whichan SiO₂ film of a first layer having a thickness ¼ of in-mediumwavelength λ is placed in close contact with the rear facet of theresonator, an amorphous Si (hereinafter described as “a-Si”) film of asecond layer having a thickness of λ/4 is superimposed thereon and anSiO₂ film of a third layer having a thickness of λ/4 is furthersuperimposed thereon.

In other words, this high reflection film is made up of a low refractiveindex film having a thickness of λ/4 in close contact with the facet, ahigh refractive index film having a thickness of λ/4 and a lowrefractive index film having a thickness of λ/4 on the rear facet of theresonator.

In this case, the electric field strength distribution in the vicinityof the rear facet of the resonator and on the reflection film becomes amaximum at the interface between the rear facet of the resonator and theSiO₂ film of the first layer, becomes a minimum at the interface betweenthe SiO₂ film of the first layer and the a-Si film of the second layer,becomes a maximum at the interface between the a-Si film of the secondlayer and the SiO₂ film of the third layer and becomes a minimum at theinterface between the SiO₂ film of the third layer and the air layer.

Since the maximum value at the interface between the a-Si film of thesecond layer and the SiO₂ film of the third layer is smaller than themaximum value at the interface between the rear facet of the resonatorand the SiO₂ film of the first layer, the electric field strength at theinterface formed of different kinds of materials becomes highest at theinterface between the rear facet of the resonator and the SiO₂ film ofthe first layer.

For example, the configuration of a high reflection film of a publiclyknown long-wavelength laser uses a five-layer structure ofSiO₂/amorphous Si/SiO₂/amorphous Si/SiO₂ on the facet formed using acleaving method, and a reflectance of 90% or more is obtained in thisway. Alternatively, a λ/4 film made of SiN is formed on the facet,amorphous Si/SiN/amorphous Si/SiN are multilayered on this and a highreflection film having a five-layer structure of SiN/amorphousSi/SiN/amorphous Si/SiN and having a reflectance of 90% is formed (e.g.,see Japanese Patent Laid-Open No. 10-290052, paragraphs 0056 and 0057).

The high reflection film in this case is composed of five layers; a lowrefractive index film having a thickness of λ/4 in close contact withthe facet, a high refractive index film having a thickness of λ/4, a lowrefractive index film having a thickness of λ/4, a high refractive indexfilm having a thickness of 2\14 and a low refractive index film having athickness of λ/4.

To reduce electric field strength at an interface between facets of aresonator, a semiconductor laser having the following configuration isknown.

In this configuration, a dielectric film having a film thickness valueof λ/4nc formed of amorphous silicon (refractive index nc to 3.5) havingsubstantially the same refractive index as that of a laser element isprovided on the light emission facet of a GaAlAs-based semiconductorlaser and a plurality of sets of low refractive index reflection filmshaving a thickness of λ/2nd made up of a dielectric film having a lowrefractive index (nd) such as an SiO₂ film and high refractive indexreflection films having a high refractive index are alternately arrangedin close contact with the dielectric film of this amorphous silicon.This configuration reduces the electric field strength of light on thelight emission facet to a minimum value (e.g., see Japanese PatentLaid-Open No. 63-220589, bottom left field and bottom right field on p.2).

Furthermore, there is disclosed an example of a semiconductor laserwhose oscillating wavelength is approximately 740 nm where an Al₂O₃ filmor SiO₂ film having an optical thickness of λ/2 is provided in contactwith an facet and multilayered pairs of TiO₂ layer and SiO₂ layer havingan optical thickness of λ/4 are sequentially arranged on the Al₂O₃ filmor SiO₂ film of λ/2; three pairs on the light emerging facet side andsix pairs on the reflection surface side, which is the opposite side(e.g., Japanese Patent Laid-Open No. 7-45910, paragraphs 0010 and 0011).

Furthermore, when multilayer coating is applied to a semiconductor laserdevice, a dielectric such as Al₂O₃, SiO₂ or Si₃N₄ is coated as anodd-numbered layer and Si is coated as an even-numbered layer, but whenSi is used for the top layer, Si is oxidized easily, and therefore thereis described an example where Si, Al₂O₃, Si and Al₂O₃ are sequentiallystacked on a cavity facet of a GaAs—GaAlAs-based semiconductor laserhaving an oscillating wavelength of 8300 Å and an Al₂O₃ layer isprovided on the top layer so as to prevent Si from being oxidized (e.g.,Japanese Patent Laid-Open No. 60-130187, from right field on p. 1 toleft field on p. 2).

Furthermore, there is disclosed an example where an AlN film is usedinstead of an Al₂O₃ film in close contact with an facet of a high outputtype semiconductor laser element having a large calorific value (e.g.,Japanese Utility Model Laid-Open No. 63-162558).

Thus, in order to reduce a threshold current, increasing a reflectanceon the facet of a resonator or increasing a normalized couplingcoefficient of a diffraction grating causes a light density on the facetof the resonator to increase. Furthermore, in the case of aconfiguration where electric field strength of light at the interfacebetween the facet of the resonator and reflection film becomes amaximum, when not only the light density on the facet of the resonatoris high but also the electric field strength of light becomes a maximum,reliability drastically deteriorates, for example, deterioration of thesemiconductor laser is more likely to occur. Furthermore, whenmechanical strength at the interface between the facet of the resonatorand reflection film is low, heating during assembly of the semiconductorlaser device may cause peeling at the interface, leading to a decreaseof yield. As such, the configuration of the above described conventionalsemiconductor laser has a difficulty in simultaneously achieving areduction of a threshold current and high reliability and may alsoresult in low yield.

In the configuration of the high reflection film of the conventionallong-wavelength semiconductor laser described above, when the highreflection film is made up of five layers; a low refractive index filmhaving a thickness of λ/4 in close contact with the facet, highrefractive index film having a thickness of λ/4, low refractive indexfilm having a thickness of λ/4, high refractive index film having athickness of λ/4 and low refractive index film having a thickness ofλ/4, as described in Japanese Patent Laid-Open No. 10-290052, there is aproblem that the electric field strength of light at the interfacebetween the facet of the resonator and the low refractive index film ofthe first layer does not always become a minimum value and the electricfield strength of light rather increases.

Furthermore, when an a-Si film is formed on the facet of the resonatoras described in Japanese Patent Laid-Open No. 63-220589, the adherenceof the semiconductor making up the resonator to a-Si is not necessarilygood. When a laser chip is assembled in a package, heat of soldering isapplied thereto, and therefore there is a problem that when theadherence of the a-Si film to the facet of the resonator isinsufficient, thermal stress applied thereto may cause the a-Si film onthe facet of the resonator to peel off. Furthermore, when a highreflection film is configured, many pairs of low refractive index filmand high refractive index film need to be superimposed one upon another,but when the adherence of the a-Si film to the facet of the resonator isinsufficient, it may be difficult to superimpose many pairs of lowrefractive index film and high refractive index film one upon another,resulting in a problem that the yield deteriorates.

Furthermore, as described in Japanese Patent Laid-Open No. 7-45910, whena high reflection film is configured by stacking multilayer pairs ofTiO₂ layer as the high refractive index film and SiO₂ layer as the lowrefractive index film, in the case of a long-wavelength laser having awavelength three times or more the wavelength described in JapanesePatent Laid-Open No. 7-45910, it is necessary to stack seven pairs ofTiO₂ film and SiO₂ film to obtain a reflectance of 80% or more in acombination of TiO₂ whose refractive index is merely on the order of 2and SiO₂ whose refractive index is 1.40 to 1.45. In this case, there isa problem that the facet coating film becomes extremely thick andpeeling of the film is more likely to occur due to thermal stress duringassembly.

SUMMARY OF THE INVENTION

The present invention has been implemented to solve the above describedproblems and it is a first object of the present invention to provide asemiconductor laser device that makes deterioration less likely to occurin the vicinity of an interface between an facet of a resonator of asemiconductor laser and a reflection film and provides a low thresholdcurrent and high yield.

According to one aspect of the present invention, there is provided asemiconductor laser device comprising: a compound semiconductor laserbody having an resonator with facets mutually opposed; a dielectric filmof any one material of SiO₂, Si₃N₄, Al₂O₃ and AlN disposed on one of thefacets of the resonator, and in close contact with the facet of theresonator, the dielectric film having a thickness obtained by dividingby 2 a positive integer multiple of a first in-medium wavelengthdetermined by a first refractive index of the material composing thedielectric film and a wavelength of emitted light of the compoundsemiconductor laser body; and a first dielectric double layered film onthe dielectric film, having a first layer of amorphous silicon and asecond layer of a material having a third refractive index lower than asecond refractive index of amorphous silicon, the first layer beingcloser to the dielectric film than the second layer, and having athickness ¼ of a second in-medium wavelength determined by the secondrefractive index and the wavelength of the emitted light, and the secondlayer having a thickness ¼ of a third in-medium wavelength determined bythe third refractive index and the wavelength of the emitted light.

Accordingly, in the semiconductor laser according to the presentinvention, the dielectric film is formed of any one material of SiO₇,Si₃N₄, Al₂O₃ and AlN, and therefore bonding strength is high at theinterface between the facet of the resonator and dielectric film and theinterface between the dielectric film and a-Si which is the first layerof the first dielectric double layered film, and it is thereby possibleto firmly stack the first dielectric double layered film and form a highreflectance film with high yield. Furthermore, the electric fieldstrength of light at the interface between the facet of the resonatorand the dielectric film and at the interface between the dielectric filmand a-Si which is the first layer of the first dielectric double layeredfilm displays a minimum value, light absorption at these interfaces issuppressed and the occurrence of COD (Catastrophic Optical Damage) canbe suppressed. Moreover, it is possible to provide a semiconductor laserdevice having a low threshold current, making deterioration less likelyto occur in the vicinity of the interface between the facet of theresonator and the reflection film and providing high reliability andhigh yield.

According to another aspect of the present invention, there is provideda semiconductor laser device comprising: a compound semiconductor laserbody having an resonator with facets mutually opposed; a dielectric filmof any one material of SiO₂, Si₃N₄ and AlN disposed on one of the facetsof the resonator, and in close contact with the facet of the resonator,the dielectric film having a thickness 1/25 or less of a first in-mediumwavelength determined by a first refractive index of the materialcomposing the dielectric film and a wavelength of emitted light of thecompound semiconductor laser body; and a first dielectric double layeredfilm on the dielectric film, having a first layer of amorphous siliconand a second layer of a material having a third refractive index lowerthan a second refractive index of amorphous silicon, the first layerbeing closer to the dielectric film than the second layer, and having athickness ¼ of a second in-medium wavelength determined by the secondrefractive index and the wavelength of the emitted light, and the secondlayer having a thickness ¼ of a third in-medium wavelength determined bythe third refractive index and the wavelength of the emitted light.

Accordingly, in the semiconductor laser according to the presentinvention, the dielectric film is formed of any one material of SiO₂,Si₃N₄ and AlN, and therefore bonding strength is high at the interfacebetween the facet of the resonator and dielectric film and the interfacebetween the dielectric film and a-Si which is the first layer of thefirst dielectric double layered film, and it is thereby possible tofirmly stack the first dielectric double layered film and form a highreflectance film with high yield. Furthermore, since the dielectric filmhas a thickness 1/25 or less of the first in-medium wavelengthdetermined by the first refractive index and the wavelength of emittedlight of the compound semiconductor laser body, by causing electricfield strength of light at the interface between the facet of theresonator and the dielectric film to approximate to a minimum value, itis possible to suppress light absorption at this interface, improve heatdissipation in the vicinity of this interface and further suppress theoccurrence of COD. Moreover, it is possible to provide a semiconductorlaser device having a low threshold current, making deterioration lesslikely to occur in the vicinity of the interface between the facet ofthe resonator and the reflection film and providing high reliability andhigh yield.

According to further aspect of the present invention, there is provideda semiconductor laser device comprising: a compound semiconductor laserbody having an resonator with facets mutually opposed; a dielectric filmof any one material of SiO₂, Si₃N₄, Al₂O₃ and AlN disposed on one of thefacets of the resonator, and in close contact with the facet of theresonator, the dielectric film having a thickness obtained by dividingby 2 a positive integer multiple of a first in-medium wavelengthdetermined by a first refractive index of the material composing thedielectric film and a wavelength of emitted light of the compoundsemiconductor laser body; and a first dielectric double layered film onthe dielectric film, having a first layer of amorphous silicon and asecond layer of a material having a third refractive index lower than asecond refractive index of amorphous silicon, the first layer beingcloser to the dielectric film than the second layer, and having athickness exceeding ¼ of a second in-medium wavelength determined by thesecond refractive index and the wavelength of the emitted light and lessthan ½ of the second in-medium wavelength, and the second layer having athickness less than ¼ of a third in-medium wavelength determined by thethird refractive index and the wavelength of the emitted light.

Accordingly, in the semiconductor laser according to the presentinvention, this configuration can reduce the electric field strength oflight at the interface between different kinds of materials constitutingthe second dielectric double layered film and further improvereliability against COD. Moreover, this configuration can provide asemiconductor laser device making deterioration less likely to occur inthe vicinity of the interface between the facet of the resonator andreflection film and having a low threshold current and high yield.

According to still further aspect of the present invention, there isprovided a semiconductor laser device comprising: a compoundsemiconductor laser body having an resonator with facets mutuallyopposed; a dielectric film of any one material of SiO₂, Si₃N₄, Al₂O₃ andAlN disposed on one of the facets of the resonator, and in close contactwith the facet of the resonator, the dielectric film having a thickness1/25 or less of a first in-medium wavelength determined by a firstrefractive index of the material composing the dielectric film and awavelength of emitted light of the compound semiconductor laser body;and a first dielectric double layered film on the dielectric film,having a first layer of amorphous silicon and a second layer of amaterial having a third refractive index lower than a second refractiveindex of amorphous silicon, the first layer being closer to thedielectric film than the second layer, and having a thickness exceeding¼ of a second in-medium wavelength determined by the second refractiveindex and the wavelength of the emitted light and less than ½ of thesecond in-medium wavelength, and the second layer having a thicknessless than ¼ of a third in-medium wavelength determined by the thirdrefractive index and the wavelength of the emitted light.

Accordingly, in the semiconductor laser according to the presentinvention, this configuration causes the dielectric film to be formed ofany one material of SiO₂, Si₃N₄, Al₂O₃ and AlN, and thereby increasesthe bonding strength at the interface between the facet of the resonatorand the dielectric film and at the interface between the dielectric filmand a-Si which is the first layer of the first dielectric double layeredfilm, can firmly stack the first dielectric double layered film and formthe high reflectance film with high yield. Furthermore, the dielectricfilm has a thickness 1/25 or less of the first in-medium wavelengthdetermined by the first refractive index and the wavelength of emittedlight of the compound semiconductor laser body, and therefore by causingthe electric field strength of light at the interface between the facetof the resonator and the dielectric film to approximate to a minimumvalue, the present embodiment can suppress light absorption at theinterface, improve heat dissipation in the vicinity of this interfaceand further suppress the occurrence of COD.

According to still further aspect of the present invention, there isprovided a semiconductor laser device comprising: a compoundsemiconductor laser body having an resonator with facets mutuallyopposed; a dielectric film of any one material of SiO₂, Si₃N₄, Al₂O₃ andAlN disposed on one of the facets of the resonator, and in close contactwith the facet of the resonator, the dielectric film having a thicknessobtained by dividing by 2 a positive integer multiple of a firstin-medium wavelength determined by a first refractive index of thematerial composing the dielectric film and a wavelength of emitted lightof the compound semiconductor laser body; and a first dielectric doublelayered film on the dielectric film, having a first layer of amorphoussilicon and a second layer of a material having a third refractive indexlower than a second refractive index of amorphous silicon, the firstlayer being closer to the dielectric film than the second layer, andhaving a thickness less than ¼ of a second in-medium wavelengthdetermined by the second refractive index and the wavelength of theemitted light, and the second layer having a thickness exceeding ¼ of athird in-medium wavelength determined by the third refractive index andthe wavelength of the emitted light and less than ½ of the thirdin-medium wavelength.

Accordingly, in the semiconductor laser according to the presentinvention, this configuration can reduce the electric field strength oflight at the interface between different kinds of materials constitutingthe second dielectric double layered film and further improvereliability against COD. Moreover, this configuration can provide asemiconductor laser device making deterioration less likely to occur inthe vicinity of the interface between the facet of the resonator andreflection film and having a low threshold current and high yield.

According to still further aspect of the present invention, there isprovided a semiconductor laser device comprising: a compoundsemiconductor laser body having an resonator with facets mutuallyopposed; a dielectric film of any one material of SiO₂, Si₃N₄, Al₂O₃ andAlN disposed on one of the facets of the resonator, and in close contactwith the facet of the resonator, the dielectric film having a thicknessof 1/25 or less of a first in-medium wavelength determined by a firstrefractive index of the material composing the dielectric film and awavelength of emitted light of the compound semiconductor laser body;and a first dielectric double layered film on the dielectric film,having a first layer of amorphous silicon and a second layer of amaterial having a third refractive index lower than a second refractiveindex of amorphous silicon, the first layer being closer to thedielectric film than the second layer, and having a thickness less than¼ of a second in-medium wavelength determined by the second refractiveindex and the wavelength of the emitted light, and the second layerhaving a thickness exceeding ¼ of a third in-medium wavelengthdetermined by the third refractive index and the wavelength of theemitted light and less than ½ of the third in-medium wavelength.

Accordingly, in the semiconductor laser according to the presentinvention, this configuration causes the dielectric film to be formed ofany one material of SiO₂, Si₃N₄, Al₂O₃ and AlN, and thereby increasesthe bonding strength at the interface between the facet of the resonatorand the dielectric film and at the interface between the dielectric filmand a-Si which is the first layer of the first dielectric double layeredfilm, can firmly stack the first dielectric double layered film and formthe high reflectance film with high yield. Furthermore, the dielectricfilm has a thickness 1/25 or less of the first in-medium wavelengthdetermined by the first refractive index and the wavelength of emittedlight of the compound semiconductor laser body, and therefore by causingthe electric field strength of light at the interface between the facetof the resonator and the dielectric film to approximate to a minimumvalue, the present embodiment can suppress light absorption at theinterface, improve heat dissipation in the vicinity of this interfaceand further suppress the occurrence of COD.

Other objects and advantages of the invention will become apparent fromthe detailed description given hereinafter. It should be understood,however, that the detailed description and specific embodiments aregiven by way of illustration only since various changes andmodifications within the scope of the invention will become apparent tothose skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a semiconductor laser according toan embodiment of the present invention.

FIG. 2 is a partial cross-sectional view in the vicinity of the fronthigh reflection film of the semiconductor laser according to anembodiment of the present invention.

FIG. 3 is a partial cross-sectional view in the vicinity of the rearhigh reflection film of the semiconductor laser according to anembodiment of the present invention.

FIG. 4 is a schematic view illustrating electric field strength of lightin the high reflection film of the semiconductor laser according toEmbodiment 1 of the present invention.

FIG. 5 is a partial cross-sectional view in the vicinity of a rear highreflection film of a semiconductor laser according to an embodiment ofthe present invention.

FIG. 6 is a schematic view illustrating electric field strength of lightin the high reflection film of the semiconductor laser according toEmbodiment 2 of the present invention.

FIG. 7 is a graph showing electric field strength at the interface withrespect to a thickness of the dielectric film according to the presentinvention.

FIG. 8 is a partial cross-sectional view in the vicinity of a rear highreflection film of a semiconductor laser according to an embodiment ofthe present invention.

FIG. 9 is a schematic view illustrating the electric field strength oflight in a high reflection film of the semiconductor laser according toEmbodiment 3 of the present invention.

FIG. 10 is a partial cross-sectional view in the vicinity of a rear highreflection film of a semiconductor laser according to an embodiment ofthe present invention.

FIG. 11 is a schematic view illustrating the electric field strength oflight in the high reflection film of the semiconductor laser accordingto Embodiment 4 of the present invention.

In all figures, the substantially same elements are given the samereference numbers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

In the following explanations, a semiconductor laser will be describedby taking a Fabry-Perot semiconductor laser as an example, but similareffects will also be obtained even when the present invention is appliedto a distributed feedback semiconductor laser or a distributed reflectorsemiconductor laser.

FIG. 1 is a cross-sectional view of a semiconductor laser according toan embodiment of the present invention. In the following drawings,identical reference numerals will denote identical or equivalentcomponents.

The cross-sectional view of the semiconductor laser 10 in FIG. 1 is across-sectional view of a cross section parallel to a waveguidedirection of the semiconductor laser and the arrow indicates emittedlight 12 of the semiconductor laser 10.

The semiconductor laser 10 is a long-wavelength semiconductor laser, anoscillating wavelength of which falls within a range of 1250 nm to 1650nm.

The semiconductor laser 10 is made up of, for example, an n-type(hereinafter, n-type will be described as “n-,” p-type will be describedas “p-” and an intrinsic type with no impurity injected in particularwill be described as “i-”) InP substrate 14, and an n-type clad layer 16of n-InP, an active layer 18 of InGaAsP and a p-type clad layer 20 ofp-InP sequentially stacked on this InP substrate 14, and the n-type cladlayer 16, active layer 18 and p-type clad layer 20 form a resonator 22.Furthermore, a p-electrode 32 is disposed on the surface of the p-typeclad layer 20 and an n-electrode 34 is disposed on the back of the InPsubstrate 14.

A semiconductor laser body 36 is made up of the InP substrate 14,resonator 22 disposed on the InP substrate 14, p-electrode 32 andn-electrode 34.

A cleaving surface including the front facet of the resonator 22 on theemitted laser light side is assumed here as a front facet 24 of thesemiconductor laser body 36 and a cleaving surface facing the frontfacet 24 via the semiconductor laser body 36 is assumed here as a rearfacet 26 of the semiconductor laser body 36. A front high reflectionfilm 28 is disposed in close contact with the surface of the front facet24 of the semiconductor laser body 36 and a rear high reflection film 30is disposed in close contact with the rear facet 26 of the semiconductorlaser body 36.

A structure with an n-AlInAs layer interposed between an n-type cladlayer of n-InP and an AlGaInAs active layer and a p-AlInAs layerinterposed between a p-type clad layer of p-InP and an AlGaInAs activelayer is known in the semiconductor laser using, for example, AlGaInAsfor the active layer as the structure of the semiconductor laser body36.

FIG. 2 is a partial cross-sectional view in the vicinity of the fronthigh reflection film of the semiconductor laser according to anembodiment of the present invention.

When the reflectance is, for example, 60%, the front high reflectionfilm 28 has the following configuration.

An SiO₂ film 40 having a film thickness of, for example, λ/2 is disposedin close contact with the front facet 24 of the semiconductor laser body36 as a dielectric film.

Here, λ is an in-medium wavelength and λ=(wavelength of light emittedfrom the semiconductor laser body in vacuum)÷(refractive index of themedium through which laser light propagates) as defined above and λvaries depending on the medium through which laser light propagates. Inthe following explanations, λ will also mean an in-medium wavelength.

Next, two pairs of dielectric double layered films 42 are stacked onthis SiO₂ film 40 as a first dielectric double layered film.

The dielectric double layered film 42 is made up of an a-Si film 42 ahaving a thickness of λ/4 as a first layer in close contact with theSiO₂ film 40 and an SiO₂ film 42 b having a thickness of λ/4 as a secondlayer superimposed on this a-Si film 42 a. When two pairs of dielectricdouble layered films 42 are superimposed one on another, the two pairsare superimposed so that the dielectric film of the first layer and thedielectric film of the second layer making up the dielectric doublelayered film 42 are arranged in the same order.

Therefore, the front high reflection film 28 is disposed on the frontfacet 24 of the semiconductor laser body 36, with the SiO₂ film 40 inclose contact with the front facet 24 of the semiconductor laser body36, a-Si film 42 a, SiO₂ film 42 b, a-Si film 42 a and SiO₂ film 42 bsuperimposed one on another in this order.

Since the refractive index of SiO₂ is 1.40 to 1.45, if the oscillatingwavelength is assumed to be 1250 nm to 1650 nm, the SiO₂ film 40 has athickness on the order of approximately 420 nm to 600 nm.

On the other hand, since the refractive index of a-Si is 3 to 3.4, thea-Si film 42 a has a thickness of approximately 90 nm to 140 nm and theSiO₂ film 42 b has a thickness on the order of approximately 210 nm to300 nm.

Furthermore, the length of the resonator 22 varies depending on the use,and is generally on the order of 200 μm to 1500 μm. The refractive indexof InP constituting the resonator 22 is on the order of 3.2 and those ofboth InGaAsP and AlGaInAs are on the order of 3.2 to 3.5. Therefore, therefractive index of the resonator 22 is substantially the same as therefractive index of the a-Si film 42 a.

FIG. 3 is a partial cross-sectional view in the vicinity of the rearhigh reflection film of the semiconductor laser according to anembodiment of the present invention.

When a reflectance is, for example, 90%, the rear high reflection film30 has the following configuration.

An SiO₂ film 40 having a layer thickness of, for example, λ/2 isdisposed as a dielectric film in close contact with the rear facet 26 ofthe semiconductor laser body 36. Next, three pairs of dielectric doublelayered films 42 are superimposed one on another on this SiO₂ film 40.

Therefore, the rear high reflection film 30 is made up of the SiO₂ film40 in close contact with the rear facet 26 of the semiconductor laserbody 36, a-Si film 42 a, SiO₂ film 42 b, a-Si film 42 a, SiO₂ film 42 b,a-Si film 42 a and SiO₂ film 42 b superimposed one on another in thisorder on the rear facet 26 of the semiconductor laser body 36.

In this configuration, the SiO₂ film 40 is used as the dielectric filmin close contact with the front facet 24 and rear facet 26 of thesemiconductor laser body 36. This can be a material that at leastdisplays high adherence to the semiconductor, has proven results and hasa smaller refractive index than the next superimposed layer (a-Si film42 a in this example). Therefore, Al₂O₃, Si₃N₄ or AlN may also be used,for example.

Furthermore, though the SiO₂ film 42 b is used as the second layer ofthe dielectric double layered film 42, this layer can also be a materialthat at least has a lower refractive index than the a-Si film 42 a, andtherefore an Al₂O₃ film may also be used instead of the SiO₂ film.

The conventional Fabry-Perot semiconductor laser has a reflectance onthe order of 30% when a single-layer Al₂O₃ film having a thickness ofλ/2 is formed on the front facet of the resonator and has a reflectanceof approximately 60% when three layers of SiO₂ film, Si film and SiO₂film each having a layer thickness of λ/4 are stacked on the rear facetof the resonator.

In this embodiment, since the reflectance of the front high reflectionfilm 28 is 60% and the reflectance of the rear high reflection film 30is 90%, mirror loss is reduced compared to the conventional Fabry-Perotsemiconductor laser and it is possible to obtain a semiconductor laserwith a low threshold current.

The reflectance of the rear high reflection film 30 is higher than thatof the front high reflection film 28, which is the facet from whichlaser light emits, and therefore the number of pairs of dielectricdouble layered films 42 superimposed increases and the total layerthickness of the rear high reflection film 30 increases, but when thetotal layer thickness exceeds 2 μm, the film is more likely to peel offdue to thermal stress during assembly. Therefore, the total layerthickness of the rear high reflection film 30 is preferably 2 μm orless.

As shown in this configuration, the a-Si film 42 a is used as the firstlayer of the dielectric double layered film 42, but since a-Si has ahigh refractive index, even if the thickness of the rear high reflectionfilm 30 is preferred to be set to 2 μm or less, it is possible to obtaina reflectance of 80% or more with a semiconductor laser in a 1.3 nm bandor 1.5 nm band.

FIG. 4 is a schematic view illustrating electric field strength of lightin the high reflection film of the semiconductor laser according toEmbodiment 1 of the present invention.

With regard to the high reflection film in FIG. 4, the rear highreflection film 30 is explained as an example, but the same applies tothe front high reflection film 28, too.

In FIG. 4, a curve 46 shows a distribution of electric field strength oflight. The upward convex portions of the curve 46 show maximum values ofthe electric field strength of light and the bottom ends show minimumvalues.

In the distribution of electric field strength of light at and in thevicinity of the rear high reflection film 30, the electric fieldstrength of light shows a minimum value on a boundary between thesemiconductor laser body 36 and the SiO₂ film 40, also shows a minimumvalue on a boundary between the SiO₂ film 40 and the a-Si film 42 a ofthe dielectric double layered film 42 of the first pair and showsminimum values on a boundary between the dielectric double layered film42 of the first pair and the dielectric double layered film 42 of thesecond pair, on a boundary between the dielectric double layered film 42of the second pair and the dielectric double layered film 42 of thethird pair and on the surface of the dielectric double layered film 42of the third pair exposed to air.

On the other hand, the electric field strength of light shows a maximumvalue on a boundary between the a-Si film 42 a of the first layer andthe SiO₂ film 42 b of the second layer of each dielectric double layeredfilms 42.

With regard to the configurations of the respective layers at and in thevicinity of the rear high reflection film 30 disposed on the rear facet26 of the semiconductor laser body 36 with respect to a refractiveindex, the rear high reflection film 30 is made up of the low refractiveindex film (SiO₂ film 40) having a layer thickness of λ/2 in closecontact with the resonator 22 of the semiconductor laser body 36, highrefractive index film (a-Si film 42 a) having a layer thickness of λ/4,low refractive index film (SiO₂ film 42 b) having a layer thickness ofλ/4, high refractive index film (a-Si film 42 a) having a layerthickness of λ/4, low refractive index film (SiO₂ film 42 b) having alayer thickness of λ/4, high refractive index film (a-Si film 42 a)having a layer thickness of λ/4 and low refractive index film (SiO₂ film42 b) having a layer thickness of λ/4.

In such a configuration, the electric field strength of light at theinterface between the resonator 22 of the semiconductor laser body 36and the low refractive index film (SiO₂ film 40) having a layerthickness of λ/2 displays a minimum value, showing the distribution ofelectric field strength of light as shown in FIG. 4.

Light absorption is essentially likely to occur on an interface betweendifferent kinds of materials. Light absorption and accompanying heatgeneration on an interface between different kinds of materials areknown to be key factors responsible for deterioration of a semiconductorlaser. COD is known widely as one of main factors for suchdeterioration, which is likely to occur due to light absorption at theinterface between the semiconductor material and the first layer of thereflection film.

To suppress light absorption on an interface between different kinds ofmaterials, reducing a photon density at that location is effective. Thisis because when a photon density is low, light absorption is less likelyto occur. The photon density is determined by electric field strength oflight and the photon density is large in an area where electric fieldstrength of light is high. Therefore, reducing electric field strengthof light on an interface between different kinds of materials iseffective in suppressing deterioration of a semiconductor laser.

Therefore, the configuration of the rear high reflection film 30according to this Embodiment 1 includes the SiO₂ film 40 in closecontact with the rear facet 26 of the semiconductor laser body 36, a-Sifilm 42 a, SiO₂ film 42 b, a-Si film 42 a, SiO₂ film 42 b, a-Si film 42a and SiO₂ film 42 b on the rear facet 26 of the semiconductor laserbody 36, constitutes a high reflection film with a high reflectance,reduces mirror loss, reduces a threshold current of the semiconductorlaser, and further reduces to a minimum the electric field strength oflight at the interface between the resonator 22 of the semiconductorlaser body 36 and the low refractive index film (SiO₂ film 40) having alayer thickness of λ/2, thereby reduces a photon density on thisinterface, reduces light absorption and accompanying heat generation,and can thereby suppress the occurrence of COD at the interface betweenthe facet of the resonator 22 of the semiconductor laser body 36 and theSiO₂ film 40. That is, it is possible to simultaneously achieve thereduction of a threshold current and suppression of the occurrence ofCOD.

Moreover, this configuration provides high adherence between thesemiconductor material making up the resonator 22 of the semiconductorlaser body 36 and the SiO₂ film 40 and high mechanical strength.Likewise, this configuration also provides high adherence at theinterface between the SiO₂ film 40 and the a-Si film 42 a and highmechanical strength. Therefore, the rear high reflection film 30 can befirmly stacked on the rear facet 26 of the semiconductor laser body 36.Thus, even when many dielectric double layered films 42 constituting therear high reflection film 30 are stacked, it is possible to suppresspeeling of the rear high reflection film 30 from the rear facet 26 ofthe semiconductor laser body 36 due to heating during assembly andincrease the yield of the product.

A case has been explained so far where a Fabry-Perot semiconductor laseris taken as an example, but it is possible to obtain effects similar tothose of the semiconductor laser 10, which is a Fabry-Perotsemiconductor laser, by using a film of material having a refractiveindex approximate to the square root of the refractive index of thematerial making up, for example, the resonator on the facet side fromwhich laser light emits, for example, an Si₃N₄ film having a layerthickness of λ/4 as an antireflection film and forming a reflection filmsimilar to the rear high reflection film 30 on the rear facet.

Especially a distributed feedback semiconductor laser has an increasedlight density on the rear high reflection film side, but can reduceelectric field strength of light at the interface between the resonatorand the rear high reflection film, and can thereby suppressdeterioration of the laser. Furthermore, the distributed feedbacksemiconductor laser can prevent the rear high reflection film frompeeling from the facet of the semiconductor laser body.

On the other hand, a distributed reflector semiconductor laser has aconfiguration in which a passive waveguide is connected on thelight-emerging facet side of the active layer contiguous to the activelayer disposed on the rear facet side of the resonator and a diffractiongrating is formed along this passive waveguide, but by forming anantireflection film on the light-emerging facet on the passive waveguideside and forming a reflection film similar to the rear high reflectionfilm 30 of Embodiment 1 on the rear facet side, it is possible to obtaineffects similar to those of the semiconductor laser 10, which is aFabry-Perot semiconductor laser.

As described above, the semiconductor laser device according to thepresent invention includes a compound semiconductor laser body having anresonator with facets mutually opposed; a dielectric film of any onematerial of SiO₂, Si₃N₄, Al₂O₃ and AlN disposed on one of the facets ofthe resonator, and in close contact with the facet of the resonator, thedielectric film having a thickness obtained by dividing by 2 a positiveinteger multiple of a first in-medium wavelength determined by a firstrefractive index of the material composing the dielectric film and awavelength of emitted light of the compound semiconductor laser body;and a first dielectric double layered film on the dielectric film,having a first layer of amorphous silicon and a second layer of amaterial having a third refractive index lower than a second refractiveindex of amorphous silicon, the first layer being closer to thedielectric film than the second layer, and having a thickness ¼ of asecond in-medium wavelength determined by the second refractive indexand the wavelength of the emitted light, and the second layer having athickness ¼ of a third in-medium wavelength determined by the thirdrefractive index and the wavelength of the emitted light.

In this configuration, the dielectric film is formed of any one materialof SiO₂, Si₃N₄, Al₂O₃ and AlN, and therefore bonding strength is high atthe interface between the facet of the resonator and dielectric film andthe interface between the dielectric film and a-Si which is the firstlayer of the first dielectric double layered film, and it is therebypossible to firmly stack the first dielectric double layered film andform a high reflectance film with high yield. Furthermore, the electricfield strength of light at the interface between the facet of theresonator and the dielectric film and at the interface between thedielectric film and a-Si which is the first layer of the firstdielectric double layered film displays a minimum value, lightabsorption at these interfaces is suppressed and the occurrence of COD(Catastrophic Optical Damage) can be suppressed. Moreover, it ispossible to provide a semiconductor laser device having a low thresholdcurrent, making deterioration less likely to occur in the vicinity ofthe interface between the facet of the resonator and the reflection filmand providing high reliability and high yield.

Second Embodiment

FIG. 5 is a partial cross-sectional view in the vicinity of a rear highreflection film of a semiconductor laser according to an embodiment ofthe present invention.

This Embodiment 2 will be explained taking a case where the presentinvention is applied to a rear high reflection film, but similar effectswill be obtained also in a case where the present invention is appliedto a front high reflection film.

In FIG. 5, a rear reflection film 50 has three pairs of dielectricdouble layered films 42 stacked one on another as in the case of therear reflection film 30 of Embodiment 1, but while in the rearreflection film 30, the SiO₂ film 40 having a layer thickness of λ/2 isdisposed in close contact with the rear facet 26 of the semiconductorlaser body 36 and three pairs of dielectric double layered films 42 aresuperimposed thereon, in the rear reflection film 50, a thin film ofSiO₂, for example, an SiO₂ film 52 having a layer thickness of λ/25 isdisposed as a dielectric film in close contact with the rear facet 26 ofthe semiconductor laser body 36 and three pairs of dielectric doublelayered films 42 are superimposed thereon.

That is, when the rear high reflection film 50 has a reflectance of, forexample, 90%, the following configuration is adopted.

The SiO₂ film 52 having a layer thickness of λ/25 is disposed in closecontact with the rear facet 26 of the semiconductor laser body 36. Next,three pairs of dielectric double layered films 42 are superimposed onthis SiO₂ film 52.

Therefore, the rear high reflection film 50 is made up of the lowrefractive index film (SiO₂ film 52) having a layer thickness of λ/25 inclose contact with the rear facet 26 of the semiconductor laser body 36,high refractive index film (a-Si film 42 a) having a layer thickness ofλ/4, low refractive index film (SiO₂ film 42 b) having a layer thicknessof λ/4, high refractive index film (a-Si film 42 a) having a layerthickness of λ/4, low refractive index film (SiO₂ film 42 b) having alayer thickness of λ/4, high refractive index film (a-Si film 42 a)having a layer thickness of λ/4 and low refractive index film (SiO₂ film42 b) having a layer thickness of λ/4 on the rear facet 26 of thesemiconductor laser body 36.

In this configuration, the SiO₂ film 52 is used as the dielectric filmin close contact with the rear facet 26 of the semiconductor laser body36. Si₃N₄, Al₂O₃ or AlN, for example, may also be used as in the case ofEmbodiment 1.

In the configuration of this Embodiment 2, the semiconductor laser body36 and the a-Si film 42 a have substantially the same order ofrefractive index and even when the thin SiO₂ film 52 is interposedbetween the semiconductor laser body 36 and a-Si film 42 a, thedistribution of electric field strength of light is determined by thespecifications of the semiconductor laser body 36 and a-Si film 42 a.

FIG. 6 is a schematic view illustrating electric field strength of lightin the high reflection film of the semiconductor laser according toEmbodiment 2 of the present invention.

In FIG. 6, the distribution of electric field strength of light in thevicinity of the rear high reflection film 50 is shown by a curve 46 andthe electric field strength of light of the rear high reflection film 50does not display a minimum value at the interface between thesemiconductor laser body 36 and the SiO₂ film 52, but displays a minimumvalue on the boundary between the SiO₂ film 52 and the a-Si film 42 a ofthe dielectric double layered film 42 of the first pair and displays aminimum value on the boundary between the dielectric double layered film42 of the first pair and the dielectric double layered film 42 of thesecond pair, on the boundary between the dielectric double layered film42 of the second pair and the dielectric double layered film 42 of thethird pair and on the surface of the dielectric double layered film 42of the third pair exposed to air. On the other hand, the electric fieldstrength of light displays a maximum value on the boundary between thea-Si film 42 a of the first layer and the SiO₂ film 42 b of the secondlayer of each dielectric double layered film 42.

Though the electric field strength of light at the interface between theresonator 22 of the semiconductor laser body 36 and low refractive indexfilm (SiO₂ film 52) having a thickness of λ/25 is not a minimum value,but since the thickness of the SiO₂ film 52 is small, this shows a valueapproximate to a minimum value on the boundary between the SiO₂ film 52and a-Si film 42 a of the dielectric double layered film 42 of the firstpair.

FIG. 7 is a graph showing electric field strength at the interface withrespect to a thickness of the dielectric film according to the presentinvention.

In FIG. 7, the horizontal axis shows a ratio of a layer thickness to anin-medium wavelength, that is, layer thickness/in-medium wavelength, andthe vertical axis shows normalized electric field strength at theinterface. When the horizontal axis is 0.25, that is, when the layerthickness is λ/4, electric field strength normalized at the interface isassumed to be 1.

That is, explaining the present embodiment using FIG. 5 and FIG. 6, itis shown that the electric field strength of light at the interfacebetween the resonator 22 of the semiconductor laser body 36 and the SiO₂film 52 is 1, which is a maximum value, when the film thickness of theSiO₂ film 52 is λ/4.

In the case of Embodiment 1, the SiO₂ film 40 having a thickness of λ/2is disposed on the facet of the resonator 22 of the semiconductor laserbody 36, but while a-Si has a high coefficient of thermal conductivity,SiO₂ has a low coefficient of thermal conductivity and there may becases where heat dissipation of heat generated at the interface betweenthe resonator 22 and the rear high reflection film 30 is not realizedsuccessfully. In such a case, the thickness of the SiO₂ film may bereduced to improve thermal conductivity.

Based on this concept, Embodiment 2 disposes the SiO₂ film 52 having areduced thickness.

As described above, the electric field strength of light does not becomea minimum value at the interface between the resonator 22 of thesemiconductor laser body 36 and the SiO₂ film 52 and the normalizedelectric field strength at the interface shown in FIG. 7 above ismaintained with the thickness of the SiO₂ film 52. Therefore, an upperlimit value of the thickness of the SiO₂ film 52 is deter mined by theextent to which the electric field strength of light is tolerable.

In the present embodiment, the layer thickness is assumed to be, forexample, λ/25 for the following reason.

In the case of a conventional Fabry-Perot semiconductor laser, the lightdensity on the rear facet of the semiconductor laser shown in Embodiment1 assumed to have a reflectance of the front facet of 60% and areflectance of the rear facet of 90% is two times higher than that of asemiconductor laser assumed to have a reflectance of the front facet of30% and a reflectance of the rear facet of 60%. Therefore, when theelectric field strength of light is reduced to ½ of that of theconventional semiconductor laser, equivalent reliability can be secured.

With reference to FIG. 7, the film thickness when the normalizedelectric field strength at the interface becomes ½ corresponds to alocation where the value of layer thickness/in-medium wavelength isroughly 0.04 and this namely shows that the layer thickness is λ/25.

In this Embodiment 2, the thickness of the SiO₂ film 52 is assumed tobe, for example, λ/25, and by setting the film thickness of the SiO₂film 52 to λ/25 or less, it is possible to further improve heatdissipation of heat generated at the interface between the resonator 22and the rear high reflection film 30 and reduce electric field strengthof light at the interface between the resonator 22 and the SiO₂ film 52.Moreover, a semiconductor laser with higher reliability against COD canbe configured.

Therefore, the configuration of the rear high reflection film 50according to Embodiment 2 includes the SiO₂ film 52 in close contactwith the rear facet 26 of the semiconductor laser body 36, a-Si film 42a, SiO₂ film 42 b, a-Si film 42 a, SiO₂ film 42 b, a-Si film 42 a andSiO₂ film 42 b superimposed one on another in this order on the rearfacet 26 of the semiconductor laser body 36, and it is thereby possibleto constitute a high reflection film with a high reflectance and reducea threshold current of the semiconductor laser with reduced mirror loss.Furthermore, by causing the electric field strength of light at theinterface between the resonator 22 of the semiconductor laser body 36and the thin low refractive index film (SiO₂ film 52) to approximate toa minimum value as much as possible, it is possible to reduce lightabsorption and accompanying heat generation, further improve heatdissipation in the vicinity of the interface between the facet of theresonator 22 of the semiconductor laser body 36 and the SiO₂ film 52 andimprove reliability against COD.

As described above, the semiconductor laser device according to thepresent invention includes a compound semiconductor laser body having anresonator with facets mutually opposed; a dielectric film of any onematerial of SiO₂, Si₃N₄ and AlN disposed on one of the facets of theresonator, and in close contact with the facet of the resonator, thedielectric film having a thickness 1/25 or less of a first in-mediumwavelength determined by a first refractive index of the materialcomposing the dielectric film and a wavelength of emitted light of thecompound semiconductor laser body; and a first dielectric double layeredfilm on the dielectric film, having a first layer of amorphous siliconand a second layer of a material having a third refractive index lowerthan a second refractive index of amorphous silicon, the first layerbeing closer to the dielectric film than the second layer, and having athickness ¼ of a second in-medium wavelength determined by the secondrefractive index and the wavelength of the emitted light, and the secondlayer having a thickness ¼ of a third in-medium wavelength determined bythe third refractive index and the wavelength of the emitted light.

In this configuration, the dielectric film is formed of any one materialof SiO₂, Si₃N₄ and AlN, and therefore bonding strength is high at theinterface between the facet of the resonator and dielectric film and theinterface between the dielectric film and a-Si which is the first layerof the first dielectric double layered film, and it is thereby possibleto firmly stack the first dielectric double layered film and form a highreflectance film with high yield. Furthermore, since the dielectric filmhas a thickness 1/25 or less of the first in-medium wavelengthdetermined by the first refractive index and the wavelength of emittedlight of the compound semiconductor laser body, by causing electricfield strength of light at the interface between the facet of theresonator and the dielectric film to approximate to a minimum value, itis possible to suppress light absorption at this interface, improve heatdissipation in the vicinity of this interface and further suppress theoccurrence of COD. Moreover, it is possible to provide a semiconductorlaser device having a low threshold current, making deterioration lesslikely to occur in the vicinity of the interface between the facet ofthe resonator and the reflection film and providing high reliability andhigh yield.

Third Embodiment

FIG. 8 is a partial cross-sectional view in the vicinity of a rear highreflection film of a semiconductor laser according to an embodiment ofthe present invention.

This Embodiment 3 will be explained taking a case where the presentinvention is applied to a rear high reflection film, but similar effectswill be obtained also in a case where the present invention is appliedto a front high reflection film.

In FIG. 8, a rear reflection film 60 includes an SiO₂ film 40 having alayer thickness of, for example, λ/2 as a dielectric film in closecontact with the rear facet 26 of the semiconductor laser body 36. Next,one pair of dielectric double layered film 62 is disposed as a seconddielectric double layered film on this SiO₂ film 40 and two pairs ofdielectric double layered films 42 are superimposed on this dielectricdouble layered film 62.

The dielectric double layered film 62 is made up of an a-Si film 62 a ofa first layer having a layer thickness of, for example, 3λ/8 in closecontact with the SiO₂ film 40 and an SiO₂ film 62 b of a second layerhaving a layer thickness of λ/8 superimposed on this a-Si film 62 a.

That is, the rear high reflection film 60 has the followingconfiguration and the reflectance changes from 90% to a certain extent.

The rear high reflection film 60 is made up of the low refractive indexfilm (SiO₂ film 40) having a layer thickness of λ/2 in close contactwith the resonator 22 of the semiconductor laser body 36, highrefractive index film (a-Si film 62 a) having a layer thickness of 3λ/8,low refractive index film (SiO₂ film 62 b) having a layer thickness ofλ/8, high refractive index film (a-Si film 42 a) having a layerthickness of λ/4, low refractive index film (SiO₂ film 42 b) having alayer thickness of λ/4, high refractive index film (a-Si film 42 a)having a layer thickness of λ/4 and low refractive index film (SiO₂ film42 b) having a layer thickness of λ/4.

FIG. 9 is a schematic view illustrating the electric field strength oflight in a high reflection film of the semiconductor laser according toEmbodiment 3 of the present invention.

In FIG. 9, the distribution of the electric field strength of light atand in the vicinity of the rear high reflection film 60 is shown by acurve 46.

In the distribution of the electric field strength of light at and inthe vicinity of the rear high reflection film 60, the electric fieldstrength of light displays a minimum value on the boundary between thesemiconductor laser body 36 and the SiO₂ film 40, also displays aminimum value on the boundary between the SiO₂ film 40 and a-Si film 62a of the dielectric double layered film 62 of the first pair anddisplays minimum values on the boundary between the dielectric doublelayered film 62 of the first pair and the dielectric double layered film42 of the second pair, on the boundary between the dielectric doublelayered film 42 of the second pair and the dielectric double layeredfilm 42 of the third pair and on the surface of the dielectric doublelayered film 42 of the third pair exposed to air. This is the same asthat of the rear high reflection film 30 of Embodiment 1.

Furthermore, the electric field strength of light displays a maximumvalue on the boundary between the a-Si film 42 a of the first layer andthe SiO₂ film 42 b of the second layer of each dielectric double layeredfilm 42. This is the same as that of the rear high reflection film 30 ofEmbodiment 1.

However, the electric field strength of light at the interface betweenthe a-Si film 62 a and the SiO₂ film 62 b of the dielectric doublelayered film 62 of the first pair is not a maximum value and is lowerthan the maximum value.

As described in the explanations of Embodiment 1, in order to suppressCOD, reducing the electric field strength of light at the interfacebetween different kinds of materials is effective. Especially in theconfigurations shown in Embodiments 1 and 2, when the electric fieldstrength of light on the boundary between the semiconductor materialmaking up the resonator 22 of the semiconductor laser body 36 and therear or front high reflection film is reduced, the interface betweendifferent kinds of materials where the electric field strength of lightis next highest is the interface between the first layer and the secondlayer of the dielectric double layered film of the first pair and is theinterface between the a-Si film 42 a and the SiO₂ film 42 b of thedielectric double layered film 42 of the first pair in the case of therear high reflection film 30 of Embodiment 1.

In order to reduce the electric field strength of light at thisinterface, Embodiment 3 adopts the high refractive index film (a-Si film62 a) having a layer thickness of 3λ/8 as the first layer in thedielectric double layered film 62 and the low refractive index film(SiO₂ film 62 b) having a layer thickness of λ/8 as the second layer toprevent these interfaces from coinciding with the position of themaximum value of the electric field strength of light.

Here, the layer thickness of the first layer is assumed to be 3λ/8 andthe layer thickness of the second layer is assumed to be λ/8, but thepresent invention is not limited to this and the first layer may have athickness exceeding λ/4 and less than λ/2 and the second layer may havea thickness less than λ/4.

Furthermore, the present embodiment assumes that the layer thickness ofthe first layer is larger and the layer thickness of the second layer issmaller, but the present invention is not limited to this and the layerthickness of the first layer may be smaller and the layer thickness ofthe second layer may be larger.

That is, similar effects can be obtained also in a configuration inwhich the first layer has a thickness less than λ/4 and the second layerhas a thickness exceeding λ/4 and less than λ/2.

Furthermore, by causing the thicknesses of the first layer and secondlayer of the dielectric double layered film 62 to change, it is possibleto select a reflectance as the entire high reflection film relativelyfreely and increase the degree of freedom of design of a high reflectionfilm.

As described above, the semiconductor laser device according to thepresent invention includes a compound semiconductor laser body having anresonator with facets mutually opposed; a dielectric film of any onematerial of SiO₂, Si₃N₄, Al₂O₃ and AlN disposed on one of the facets ofthe resonator, and in close contact with the facet of the resonator, thedielectric film having a thickness obtained by dividing by 2 a positiveinteger multiple of a first in-medium wavelength determined by a firstrefractive index of the material composing the dielectric film and awavelength of emitted light of the compound semiconductor laser body;and a first dielectric double layered film on the dielectric film,having a first layer of amorphous silicon and a second layer of amaterial having a third refractive index lower than a second refractiveindex of amorphous silicon, the first layer being closer to thedielectric film than the second layer, and having a thickness exceeding¼ of a second in-medium wavelength determined by the second refractiveindex and the wavelength of the emitted light and less than ½ of thesecond in-medium wavelength, and the second layer having a thicknessless than ¼ of a third in-medium wavelength determined by the thirdrefractive index and the wavelength of the emitted light.

Furthermore, the semiconductor laser device according to the presentinvention includes a compound semiconductor laser body having anresonator with facets mutually opposed; a dielectric film of any onematerial of SiO₂, Si₃N₄, Al₂O₃ and AlN disposed on one of the facets ofthe resonator, and in close contact with the facet of the resonator, thedielectric film having a thickness obtained by dividing by 2 a positiveinteger multiple of a first in-medium wavelength determined by a firstrefractive index of the material composing the dielectric film and awavelength of emitted light of the compound semiconductor laser body;and a first dielectric double layered film on the dielectric film,having a first layer of amorphous silicon and a second layer of amaterial having a third refractive index lower than a second refractiveindex of amorphous silicon, the first layer being closer to thedielectric film than the second layer, and having a thickness less than¼ of a second in-medium wavelength determined by the second refractiveindex and the wavelength of the emitted light, and the second layerhaving a thickness exceeding ¼ of a third in-medium wavelengthdetermined by the third refractive index and the wavelength of theemitted light and less than ½ of the third in-medium wavelength.

In addition to the effects of Embodiment 1, this configuration canreduce the electric field strength of light at the interface betweendifferent kinds of materials constituting the second dielectric doublelayered film and further improve reliability against COD. Moreover, thisconfiguration can provide a semiconductor laser device makingdeterioration less likely to occur in the vicinity of the interfacebetween the facet of the resonator and reflection film and having a lowthreshold current and high yield.

Fourth Embodiment

FIG. 10 is a partial cross-sectional view in the vicinity of a rear highreflection film of a semiconductor laser according to an embodiment ofthe present invention.

This Embodiment 4 will be explained taking a case where the presentinvention is applied to a rear high reflection film, but similar effectswill be obtained also in a case where the present invention is appliedto a front high reflection film.

In FIG. 10, a rear reflection film 70 is provided with an SiO₂ film 52having a layer thickness of, for example, λ/25 as a dielectric film inclose contact with the rear facet 26 of the semiconductor laser body 36.Next, one pair of dielectric double layered film 62 is superimposed onthis SiO₂ film 52 and two pairs of dielectric double layered films 42are superimposed on this dielectric double layered film 62.

That is, the rear high reflection film 70 has the followingconfiguration and the reflectance thereof changes from 90% to a certainextent.

The rear high reflection film 70 is made up of the low refractive indexfilm (SiO₂ film 52) having a layer thickness of λ/25 in close contactwith the resonator 22 of the semiconductor laser body 36, highrefractive index film (a-Si film 62 a) having a layer thickness of 3λ/8,low refractive index film (SiO₂ film 62 b) having a layer thickness ofλ/8, high refractive index film (a-Si film 42 a) having a layerthickness of λ/4, low refractive index film (SiO₂ film 42 b) having alayer thickness of λ/4, high refractive index film (a-Si film 42 a)having a layer thickness of λ/4 and low refractive index film (SiO₂ film42 b) having a layer thickness of λ/4.

FIG. 11 is a schematic view illustrating the electric field strength oflight in the high reflection film of the semiconductor laser accordingto Embodiment 4 of the present invention.

In FIG. 11, the distribution of the electric field strength of light atand in the vicinity of the rear high reflection film 70 is shown by acurve 46.

In the distribution of the electric field strength of light at and inthe vicinity of the rear high reflection film 70, the electric fieldstrength of light does not display a minimum value at the interfacebetween the semiconductor laser body 36 and the SiO₂ film 52, displays aminimum value on the boundary between the SiO₂ film 52 and a-Si film 62a of the dielectric double layered film 62 of the first pair anddisplays minimum values on the boundary between the dielectric doublelayered film 62 of the first pair and the dielectric double layered film42 of the second pair, the boundary between the dielectric doublelayered film 42 of the second pair and the dielectric double layeredfilm 42 of the third pair and the surface of the dielectric doublelayered film 42 of the third pair exposed to air.

Furthermore, the electric field strength of light displays maximumvalues on the boundary between the a-Si film 42 a of the first layer andthe SiO₂ film 42 b of the second layer of the dielectric double layeredfilm 42 of the second pair and third pair, but the electric fieldstrength of light at the interface between the a-Si film 62 a and theSiO₂ film 62 b of the dielectric double layered film 62 of the firstpair is not a maximum value but is lower than the maximum value.

The SiO₂ film 52 in close contact with the facet 26 of the resonator 22of the semiconductor laser body 36 has a smaller coefficient of thermalconductivity than that of a-Si, and therefore the thinner the film, thebetter the heat dissipation thereof. The layer thickness of the SiO₂film 52 is preferably set to λ/25 or less as described in Embodiment 2.

This configuration has the effects described in Embodiment 2.

Furthermore, the electric field strength of light at the interfacebetween the a-Si film 62 a and SiO₂ film 62 b of the dielectric doublelayered film 62 of the first pair is not a maximum value but lower thanthe maximum value. When the electric field strength of light on theboundary between the semiconductor material making up the resonator 22of the semiconductor laser body 36 and the rear or front high reflectionfilm is reduced, the interface between different kinds of materialswhere the electric field strength of light is the next highest is theinterface between the first layer and second layer of the dielectricdouble layered film of the first pair and Embodiment 3 adopts the highrefractive index film (a-Si film 62 a) having a layer thickness of 3λ/8for the first layer and the low refractive index film (SiO₂ film 62 b)having a layer thickness of λ/8 for the second layer of the dielectricdouble layered film 62 to reduce the electric field strength of light atthis interface, and thereby prevents these interfaces from coincidingwith the positions of the maximum values of the electric field strengthof light.

The thickness of the a-Si film 62 a and SiO₂ film 62 b of the dielectricdouble layered film 62 of the first pair is determined as described inEmbodiment 3.

Therefore, the invention according to Embodiment 4 also has the effectsof Embodiment 3 in addition to the effects of Embodiment 2.

As described above, the semiconductor laser device according to thepresent invention includes a compound semiconductor laser body having anresonator with facets mutually opposed; a dielectric film of any onematerial of SiO₂, Si₃N₄, Al₂O₃ and AlN disposed on one of the facets ofthe resonator, and in close contact with the facet of the resonator, thedielectric film having a thickness of 1/25 or less of a first in-mediumwavelength determined by a first refractive index of the materialcomposing the dielectric film and a wavelength of emitted light of thecompound semiconductor laser body; and a first dielectric double layeredfilm on the dielectric film, having a first layer of amorphous siliconand a second layer of a material having a third refractive index lowerthan a second refractive index of amorphous silicon, the first layerbeing closer to the dielectric film than the second layer, and having athickness exceeding ¼ of a second in-medium wavelength determined by thesecond refractive index and the wavelength of the emitted light and lessthan ½ of the second in-medium wavelength, and the second layer having athickness less than ¼ of a third in-medium wavelength determined by thethird refractive index and the wavelength of the emitted light.

Furthermore, the semiconductor laser device according to the presentinvention includes a compound semiconductor laser body having anresonator with facets mutually opposed; a dielectric film of any onematerial of SiO₂, Si₃N₄, Al₂O₃ and AlN disposed on one of the facets ofthe resonator, and in close contact with the facet of the resonator, thedielectric film having a thickness of 1/25 or less of a first in-mediumwavelength determined by a first refractive index of the materialcomposing the dielectric film and a wavelength of emitted light of thecompound semiconductor laser body; and a first dielectric double layeredfilm on the dielectric film, having a first layer of amorphous siliconand a second layer of a material having a third refractive index lowerthan a second refractive index of amorphous silicon, the first layerbeing closer to the dielectric film than the second layer, and having athickness less than ¼ of a second in-medium wavelength determined by thesecond refractive index and the wavelength of the emitted light, and thesecond layer having a thickness exceeding ¼ of a third in-mediumwavelength determined by the third refractive index and the wavelengthof the emitted light and less than ½ of the third in-medium wavelength.

This configuration causes the dielectric film to be formed of any onematerial of SiO₂, Si₃N₄, Al₂O₃ and AlN, and thereby increases thebonding strength at the interface between the facet of the resonator andthe dielectric film and at the interface between the dielectric film anda-Si which is the first layer of the first dielectric double layeredfilm, can firmly stack the first dielectric double layered film and formthe high reflectance film with high yield. Furthermore, the dielectricfilm has a thickness 1/25 or less of the first in-medium wavelengthdetermined by the first refractive index and the wavelength of emittedlight of the compound semiconductor laser body, and therefore by causingthe electric field strength of light at the interface between the facetof the resonator and the dielectric film to approximate to a minimumvalue, the present embodiment can suppress light absorption at theinterface, improve heat dissipation in the vicinity of this interfaceand further suppress the occurrence of COD.

Furthermore, the present embodiment can reduce the electric fieldstrength of light at the interface between different kinds of materialsmaking up the second dielectric double layered film and further improvereliability against COD. Moreover, it is possible to provide asemiconductor laser device having a low threshold current, makingdeterioration less likely to occur in the vicinity of the interfacebetween the facet of the resonator and the reflection film and providehigh reliability and high yield.

The above described embodiments allow COD to be suppressed, therebyimprove durability, reduce a threshold current leading to a reduction ofenergy consumption, improve yield and provide the effects of reducingenvironment loads or the like.

As described above, the semiconductor laser device according to thepresent invention is suitable for use in an optical communication systemor the like.

While the presently preferred embodiments of the present invention havebeen shown and described. It is to be understood these disclosures arefor the purpose of illustration and that various changes andmodifications may be made without departing from the scope of theinvention as set forth in the appended claims.

1-5. (canceled)
 6. A semiconductor laser device comprising: a compoundsemiconductor laser body emitting light and having a resonator withfacets mutually opposed to each other; a dielectric film, selected fromthe group consisting of SiO₂, Si₃N₄, Al₂O₃, and AlN, disposed on one ofthe facets of the resonator, and in contact with the facet of theresonator, wherein the dielectric film has a thickness 1/25 or less of afirst in-medium wavelength, determined by a first refractive index,which is the refractive index of the material composing the dielectricfilm, and wavelength of the light emitted by the compound semiconductorlaser body; and a first dielectric double layered film on the dielectricfilm, having a first layer of amorphous silicon and a second layer of amaterial having a third refractive index, lower than a second refractiveindex, which is the refractive index of amorphous silicon, wherein thefirst layer is closer to the dielectric film than the second layer, andhas a thickness exceeding ¼ of a second in-medium wavelength, determinedby the second refractive index and the wavelength of the light emitted,and less than ½ of the second in-medium wavelength, and the second layerhas a thickness less than ¼ of a third in-medium wavelength, determinedby the third refractive index and the wavelength of the light emitted.7. (canceled)
 8. A semiconductor laser device comprising: a compoundsemiconductor laser body emitting light and having a resonator withfacets mutually opposed to each other; a dielectric film, selected fromthe group consisting of SiO₂, Si₃N₄, Al₂O₃, and AlN, disposed on one ofthe facets of the resonator, and in contact with the facet of theresonator, wherein the dielectric film has a thickness of 1/25 or lessof a first in-medium wavelength, determined by a first refractive index,which is the refractive index of the material composing the dielectricfilm, and wavelength of the light emitted by the compound semiconductorlaser body; and a first dielectric double layered film on the dielectricfilm, having a first layer of amorphous silicon and a second layer of amaterial having a third refractive index, lower than a second refractiveindex, which is the refractive index of amorphous silicon, wherein thefirst layer is closer to the dielectric film than the second layer, andhas a thickness less than ¼ of a second in-medium wavelength, determinedby the second refractive index and the wavelength of the light emitted,and the second layer has a thickness exceeding ¼ of a third in-mediumwavelength, determined by the third refractive index and the wavelengthof the light emitted, and less than ½ of the third in-medium wavelength.9. (canceled)
 10. The semiconductor laser device according to claim 6,further comprising at least one second dielectric double layered filmdisposed on the first dielectric double layered film, wherein the seconddielectric double layered film has a first layer of amorphous siliconand a second layer of a material having a fourth refractive index, lowerthan the second refractive index, the first layer is closer to thedielectric film than the second layer, and has a thickness ¼ of thesecond in-medium wavelength, determined by the second refractive indexand the wavelength of the light emitted, and the second layer has athickness ¼ of a fourth in-medium wavelength, determined by the fourthrefractive index and the wavelength of the light emitted.
 11. (canceled)12. The semiconductor laser device according to claim 8, furthercomprising at least one second dielectric double layered film disposedon the first dielectric double layered film, wherein the seconddielectric double layered film has a first layer of amorphous siliconand a second layer of a material having a fourth refractive index, lowerthan the second refractive index, the first layer is closer to thedielectric film than the second layer, and has a thickness ¼ of thesecond in-medium wavelength, determined by the second refractive indexand the wavelength of the light emitted, and the second layer has athickness ¼ of a fourth in-medium wavelength, determined by the fourthrefractive index and the wavelength of the light emitted.
 13. (canceled)14. The semiconductor laser device according to claim 6, furthercomprising at least one second dielectric double layered film disposedon the first dielectric double layered film, wherein the seconddielectric double layered film has a first layer of amorphous siliconand a second layer of a material having a fourth refractive index, lowerthan the second refractive index, the first layer is closer to thedielectric film than the second layer, and has a thickness exceeding ¼of the second in-medium wavelength, determined by the second refractiveindex and the wavelength of the light emitted, and less than ½ of thesecond in-medium wavelength, and the second layer has a thickness lessthan ¼ of a fourth in-medium wavelength, determined by the fourthrefractive index and the wavelength of the light emitted.
 15. (canceled)16. The semiconductor laser device according to claim 8, furthercomprising at least one second dielectric double layered film disposedon the first dielectric double layered film, wherein the seconddielectric double layered film has a first layer of amorphous siliconand a second layer of a material having a fourth refractive index, lowerthan the second refractive index, the first layer is closer to thedielectric film than the second layer, and has a thickness exceeding ¼of the second in-medium wavelength, determined by the second refractiveindex and the wavelength of the light emitted, and less than ½ of thesecond in-medium wavelength, and the second layer has a thickness lessthan ¼ of a fourth in-medium wavelength, determined by the fourthrefractive index and the wavelength of the light emitted.
 17. (canceled)18. The semiconductor laser device according to claim 6, furthercomprising at least one second dielectric double layered film disposedon the first dielectric double layered film, wherein the seconddielectric double layered film has a first layer of amorphous siliconand a second layer of a material having a fourth refractive index, lowerthan the second refractive index, the first layer is closer to thedielectric film than the second layer, and has a thickness less than ¼of the second in-medium wavelength, determined by the second refractiveindex and the wavelength of the light emitted, and the second layer hasa thickness exceeding ¼ of a fourth in-medium wavelength, determined bythe fourth refractive index and the wavelength of the light emitted, andless than ½ of the fourth in-medium wavelength.
 19. (canceled)
 20. Thesemiconductor laser device according to claim 8, further comprising atleast one second dielectric double layered film disposed on the firstdielectric double layered film, wherein the second dielectric doublelayered film has a first layer of amorphous silicon and a second layerof a material having a fourth refractive index, lower than the secondrefractive index, the first layer is closer to the dielectric film thanthe second layer, and has a thickness less than ¼ of the secondin-medium wavelength, determined by the second refractive index and thewavelength of the light emitted, and the second layer has a thicknessexceeding ¼ of a fourth in-medium wavelength, determined by the fourthrefractive index and the wavelength of the light emitted, and less than½ of the fourth in-medium wavelength.